1. Field of the Invention
The present invention relates to antennas whose dimensions may be modified so as to modify their radioelectric characteristics with the objective, generally, of making them work, by choice, in one out of several frequency bands; these antennas are mainly used at frequencies below 1 GHz.
2. Discussion of the Background
Antennas are known which comprise at least one radiating element having variable electrical length, which is constructed from an array of n, with n an integer greater than 1, conducting sections separated by switching modules designed to electrically link together all or some of the sections. Onwards of that of its two ends where it is fed, the array constitutes a radiating element which is made, by choice, of 1, 2, . . . n sections. This is the way in which, in particular, variable geometry unipole antennas are constructed.
In certain cases the switching modules consist simply of fixing means, generally a screw and a nut carried respectively by the opposite ends of the sections to be joined electrically. When joining is to be controlled remotely for reasons of ease and/or speed of implementation, it is a known practice to use a switching module of the electrical relay type and linking means made of two electrical wires for controlling the relay; the presence of these wires which are radioelectrically insulated from the conducting sections in a more or less effective manner, limits the performance of the antenna and this limitation is all the greater when the antenna caters for high powers or high voltages, as is the case with antennas operating with decameter waves.
It should also be noted that it is known practice, through U.S. Pat. No. 4,728,805, to construct antennas from a three-dimensional matrix whose rows are made of conducting segments with, at the intersections of the rows, photoconducting elements which provide connections between segments when they are illuminated. On account of the photoconductors available on the market, this technique can only be used with low-power antennas and, moreover, these photoconductors must be permanently illuminated in order to be maintained in the on state.